The invention is related to the field of controllers for direct current (DC) motors, such as motor controllers used in conjunction with voice coil motors used for read/write head positioning in disk drives.
In disk drives, it is common to control the motion of a read/write head across a recording medium by a type of direct-current (DC) motor known as a voice-coil motor or VCM. The operation of the VCM is in turn controlled by a control circuit referred to as a motor controller. Typically a form of feedback is used to enable the controller to precisely control head position and speed during normal operation of the disk drive. Data retrieved from the disk provides an indication of radial head position. This indication is used by the motor controller to compare actual head position and speed with desired values. The resulting difference values are used to modify the current flowing through the VCM winding, thus changing the head position and/or speed by changing the rotational speed of the VCM.
The above-described control technique using data feedback cannot operate when the head is not engaging the medium. This situation occurs, for example, when the head is being loaded onto the disk from a resting ramp, and when the head is being retracted from the disk onto the resting ramp. Accurate control of head speed is desired even during these periods in which the head is not engaging the medium. Such control minimizes the chances that either the heads or head stops adjacent to the ramps will be damaged.
To achieve this control, some prior-art motor controllers employ a microprocessor-based speed control circuit that is capable of accurately controlling motor speed without receiving data retrieved from the medium by the read/write head. Instead, the circuit operates by detecting the back electromagnetic force (back EMF or BEMF) of the VCM and then applying a complex speed control algorithm to generate the desired VCM current.
It is known to obtain a finite amount of electrical power from the a motor during periods in which normal operating power is lost but the motor is still spinning. In some disk drives this is done with the spindle motor. In effect, the motor is converted to use as a generator during these periods. For example, in one common scheme the motor's back EMF causes diodes in the motor controller's drive circuits to become forward biased, so that current flows from the motor coil into a power bus from which the drive signal portion of the motor control circuitry is powered. This residual power is used for special functions needed during such times. A common such feature is braking of the motor.
Head-positioning VCMs typically cannot generate enough power to operate a microprocessor and associated circuitry during power-loss periods. Therefore in microprocessor-based controllers, the microprocessor and related circuitry are usually powered separately from the power bus used for the motor drive signals. As a result, the microprocessor circuitry becomes inoperable when power is lost, and therefore the microprocessor-based speed control loop becomes inoperable as well.
It is important to accurately control head motion during loading and retraction even under power loss conditions. In fact, it is a normal operating feature of disk drives to retract the head when power is first lost, in order to avoid corrupting data on the disk. In the above-described prior motor controllers, however, the desired accurate control cannot be provided by the microprocessor-based control circuit when power to the disk drive is lost.